Generally, in internal combustion engines and compressors for automobiles and the like, an assembly consisting of a crankshaft and a connecting rod is used for converting translational reciprocal motion to rotational motion or rotational motion to translational reciprocal motion. In this assembly, the crankshaft is usually rotated in a main journal section in the direction of the arrow in FIG. 1. The main journal section is rotatably supported in a pair of semi-cylindrical sliding bearing halves mounted in a bore of a circular sectional profile assembled in a housing. Further, an end of an adjacent crankshaft is coupled to the journal section, in which one end of a connecting rod is rotatably supported via a pair of semi-cylindrical sliding bearing halves. In the lubricant supply system of this assembly, a main oil supply passages and communicating passages are formed in the housing, and through holes are formed in the individual sliding bearing halves and communicated with communicating passages and the main supply passage. The main journal section of the crankshaft is provided with passages extending in the radial and oblique directions, and the lubricant is supplied through these passages to an oil port in the journal section at the end to be supplied from this oil port to the sliding bearing for lubrication of the connecting rod system.
More specifically, in the assembly consisting of the crankshaft and connecting rod, the lubricant is supplied to the oil supply passage of housing 6 from, for instance, a main oil pump under a predetermined pressure. In the housing, the lubricant from the main oil passage is supplied through the communicating passages to the through holes in the sliding bearing halves and thence to a space between the outer periphery of the crankshaft and inner periphery of each sliding bearing half. The main journal section of the crankshaft is rotatably supported in a state floated by an oil film formed between the outer and inner peripheries. Meanwhile, the lubricant flowing through the space between the crankshaft outer periphery and bearing surface of the sliding bearing is supplied through a radial passage in the journal section and then through an oblique passage to a connecting rod support section. In the connecting rod support section, like the case of the crankshaft, the lubricant is supplied from the oil port to a space between the bearing surface of the sliding bearing mounted on the connecting rod and the outer periphery of the journal section at the end, thus providing predetermined lubricating and cooling effects.
More specifically, in the journal section of the crankshaft and connecting rod (hereinafter referred to as crankshaft or on the like) an oil film formed between the outer periphery of the crankshaft or the like and sliding bearing receives repetitive load on the journal section of the crankshaft or the like and in this way the journal section is rotatably supported. At this time, a continuous and uniform oil film has to be present at all time. This is so because if the oil film is broken or is absent while the journal section of the crankshaft or the like is rotating in the sliding bearing, the outer periphery of the journal section comes into metal contact with the surface of the sliding bearing, thus giving rise to considerate wear or seizure.
Further, recently there has been a trend for an increase of the compressor output and operating load of vehicles. This trend leads to reducing the thickness of the oil film between, the crankshaft or the like and sliding bearing. Accordingly, metal contact has been observed in place, in which no metal contact has heretofore occurred.
Heretofore, improvements in the chemical composition of the bearing metals or alloys forming the bearing surface of the sliding bearing and improvements of the structure of the crankshaft or the like have been proposed from a consideration that such improvements will greatly influence the nature and behavior of the oil film thus formed. Among the former improvements there are those which permit considerable lubrication to be maintained even by interface lubrication which is subject to metal contact. However, the latter structural improvements do not always cope with the recent trend for increasing output and load concerning automobiles.
The structural improvements of the crankshaft or the like can be roughly classified into those which provide an adequate clearance between the bearing surface of the sliding bearing and outer periphery of the crankshaft or the like and those which provide the bearing surface of the sliding bearing with an oil groove, in which the lubricant is temporarily collected before being dispersed over the bearing surface. In the former case, with the provision of the clearance the lubrication property is improved to obtain a cooling effect. However, if the clearance is excessive, the oil film pressure is increased, leading to rupture of the bearing. Therefore, the clearance can not be excessively increased from the standpoint of the bearing life. For this reason, a split bearing has been proposed, which has a cylindrical sliding surface constituted by a crown surface equal to the radius of the crankshaft or the like to be supported and a clearance surface having a radius greater than the radius of the crown surface, as disclosed in U.S. Pat. No. 4,311,34. With this split bearing consisting of a pair of bearing halves, under a high load condition the rotating crankshaft or the like approaches the crown surface of one of the split bearing halves and becomes more distant from the crown surface of the other split bearing half, thus forming a predetermined clearance. The lubricant flows through the clearance to provide lubricating and cooling effects. However, even where such a clearance is provided, under recent high-output, high-load operating conditions the oil film pressure is liable to be reduced greatly in the neighborhood of the oil port of the connecting rod, thus giving rise to metal contact to cause seizure or abnormal wear of the bearing.
Further, where the bearing surface of the sliding bearing is formed with an oil groove, the lubricant supplied is once captured in the oil groove to be distributed over the bearing surface, so that the lubricant dispersion property is greatly enhanced. More specifically, as shown in U.S. Pat. No. 2,631,905, the bearing surface of the sliding bearing is provided with an oil groove, an oil supply groove is communicated with the oil film, and a lubricant supply system is communicated with the oil supply groove. With this sliding bearing, the lubricant supplied form the oil supply system is totally supplied through the oil supply groove to the oil groove, collected temporarily in this oil groove and then dispersed over the bearing surface. However, where such an oil groove is formed on the bearing surface, it is difficult for the oil groove forming portion to support the load. And also, there arises a tendency of a sudden reduction of the oil film thickness, on the oil groove forming portion, where a breakage of the oil film is liable to occur. Particularly, this trend is pronounced under high-output, high-load operating conditions, and it is said that in an assembly consisting of the crankshaft and connecting rod it is generally undesired to provide the bearing surface of the sliding bearing with an oil groove with the sole aim of a lubricant dispersion property.
Further, it has been proposed to provide a shallow groove for the sake of improving the lubricant dispersion property. However, the oil groove is so shallow that it is free from the problems noted above, the aims of dispersion and cooling of the lubricant can hardly be attained.